Many electronic devices use switched mode power supplies due to their higher power efficiency and smaller physical size compared to other types of power supply. Switched mode power supplies are also advantageous in that these supplies can provide regulated output voltages having magnitudes higher than or lower than the unregulated input supply and can also supply different outputs of different voltages.
In a switched mode power supply system, a controller integrated circuit (IC) is used to generate the switching signal. To power the controller IC, different schemes can be found. U.S. Pat. No. 6,587,357, which is incorporated herein by reference, describes one known switched mode power supply system and method of powering the controller IC and is shown in FIG. 1. Referring to FIG. 1, the switched mode power supply includes a primary circuit portion 20, a secondary circuit portion 30, a self-supply power controller 40 and a feedback circuit 38. The primary circuit portion 20 includes a power source 22 and a primary inductive winding 24 coupled to the power source 22 at one end and to the self-supply power controller 40 at the other end. The secondary circuit portion 30 includes a secondary inductive winding 32 (coupled to the primary inductive winding 24), a rectifier 34 and a filter capacitor 36. The self-supply power controller 40 includes a controllable power source 42, a comparator circuit 44, a controller 48, a power switch 52 and a reference supply voltage 56. If the power controller 40 is implemented in a single IC package, four pins generally are required, shown as P1, P2, P3 and P4.
During operation, the comparator 44 compares the voltage on supply line 54 with the reference voltage 56. Specifically, if the voltage on supply line 54 is less than or equal to a first voltage magnitude, the comparator 44 enables the controllable power source 42, which charges up the capacitor 46. As a result, the voltage on supply line 54 increases. On the other hand, if the voltage on supply line 54 is more than or equal to a second voltage magnitude, the comparator 44 disables the controllable power source 42. Due to the current consumption of the controller, the voltage on supply line 54 will fall.
The operation of the prior art arrangement of FIG. 1 is shown in FIG. 2. Between t0 and t1, the capacitor 46 is charging up, because the voltage on supply line 54 is lower than the first voltage magnitude so the output of the comparator 44 enables the controllable power source 42. At t1, when the voltage on supply line 54 reaches the second voltage magnitude, the comparator output disables the controllable power source 42. Due to the current consumption of the controller, the voltage on supply line 54 will fall. At t2, when the voltage on supply line 54 falls to the first voltage magnitude, the comparator circuit output enables the controllable power source, so that the capacitor starts to charge up again. Thus, the voltage on supply line 54 (which is the voltage across capacitor 46) fluctuates between the first and second voltage magnitudes during the whole period of operation. This cycle repeats until power is removed from the input of the primary circuit portion 20.
Note that, in the arrangement of U.S. Pat. No. 6,587,357, the controllable power source is either enabled or disabled. When it is enabled, there is a constant current flowing to charge up the capacitor. When it is disabled, the capacitor discharges due to consumption by the self-supply power controller 40.
Whilst the arrangement of U.S. Pat. No. 6,587,357, illustrated in FIGS. 1 and 2, is simple and easy to implement, it does have a number of disadvantages. Primarily, this type of self-supply power is quite inefficient because the power controller IC is always powered by the controllable power source, which is coupled to the high bus voltage (usually around 400V), and power consumption in the self-supply power source is quite high. Additionally, the high power consumption may cause thermal problems for the controller IC.
U.S. Pat. No. 5,812,383, which is incorporated herein by reference, shows another known switched mode power supply system and method of powering the controller IC and is shown in FIG. 3. FIG. 4 shows the variation of VAUX (equivalent to VCC in U.S. Pat. No. 6,587,357) with time and FIG. 5 shows variation of the voltage across the control output capacitor, VAUX, burst mode signal S6 and the drive signal with time.
In the arrangement shown in FIG. 3, during normal operation the power supply is supplied by the auxiliary winding, and during standby mode the power supply is supplied by the self-supply current source. Therefore, there is no power consumption in the self-supply current source during normal operation. However, one major disadvantage of the U.S. Pat. No. 5,812,383 arrangement is that the output voltage is not well regulated during burst mode operation since an auto-restart concept is used, i.e., it is as if the power supply is starting up again in every single cycle. Because of this, the load response of the power supply during burst mode operation is poor.